The nature of the blockade of 5-hydroxymethylcytosine (5-HMC) synthesis in the brain of rats on a low protein diet will be investigated, beginning with the study of 5-HMC formation from the methyl group of thymine and the cytosine moiety of DNA. Normal rats will be used as controls to gain insight into the step(s) which are defective in the malnourished animal. The turnover of 5-HMC in vivo will be studied in the brain DNA of the normal and malnourished rat as an additional means of ascertaining possible metabolic differences which may be lost in the in vitro systems. The subcellular distribution the brain DNA which contains 5-HMC will be examined, with particular reference to synaptosome subfractions. We have found certain of the micro methods to be unreliable in quantitative determination of DNA in fractions high in lipid and sialic acid. A micro method will be developed for DNA estimation in these particulate components of the synaptosome.